The present invention relates to an electrically conductive ceramic material which can be used under corrosive or oxidizing environmental conditions and even at temperatures around 1300 degrees C.
High temperature vapor phase electrolysis or the operation of high temperature fuel cells employment of low resistive electrodes having a low transition resistance to the electrolyte. These electrodes have to maintain their properties even in an oxidizing environment as well as in a high temperature environment. Aside from resistance against high temperature and agression from oxidizing gases the particular material involved should have as high an electrical conductivity as possible. It should also be not too expensive and it should be able to withstand cyclically occurring thermal loads. In addition these electrodes should not be prone to develop high resistive (ohmic) surface strata or coatings because only low voltages are to be extracted from these electrodes.
The known noble metals fulfill all but one of these requirements and it is obvious on its face that they are quite expensive and therefore do not meet the particular requirement of eceonomy. Alternatively ceramic materials are known which conduct electricity quite well. Known here are molybdenum silicite or silicon carbide. Unfortunately under oxidizing conditions these materials develop high ohmic surface covers or coatings just like conventional heating conductor alloys. Also, the transition resistance between electrode and electrolyte is so high that for low voltage drops within the system the electrodes pose unusable conditions.
Another alternative to noble metals are the so called conductive or electrically conductive oxide ceramic materials. From a conductivity point of view they constitute a second choice because their conductivity generally is quite below the conductivity of noble metals. On the other hand this kind of ceramic is of advantage if within a particular construction and compound arrangement involving other ceramic materials one can provide matching of the respective thermal coefficient of extension. Known members of this type of electrically conductive ceramics are the mixed oxides which crystallize in a lattice structure known as Perowskit lattice, wherein a transition material is embedded in a cation substrate lattice--the so called B lattice, is the transition metal having two different levels. High electrical conductivity is observed in the quasi binary systems such as LaMnO.sub.3 --CaMnO.sub.3 ; LaCoO.sub.3 --SrCoO.sub.3 ; LaCrO.sub.3 --LaNiO.sub.3 or LaNiO.sub.3 --CaNiO.sub.3.
German printed patent application No. 27 35 934 (see also U.S. Pat. No. 4,197,362) discloses a material having a composition which can generally be described as La.sub.x Ca.sub.1-x MnO.sub.3+.DELTA.. These materials are very good conductors. The quantity delta adjusts itself for a given composition of the cation as a function of the partial pressure of oxygen and of the temperature and varies depending on the environmental conditions between about -0.25 and +0.25.